Concentric conducting system



Nov. 11, 1930. NEIN 1,781,124

CONCENTRIC CONDUCTING SYS TEM Filed May 23. 1929 2 Sheets-Sheet l I? Q a0 ,0

A 1 10 1 ma! EMF INVENTOR ATTORNEY Nov. 11, 1930. H. R. NEIN 1,781,124

CONCENTRIC CONDUCTING SYSTEM Filed May 23. 1929 2 Sheets-Sheet 2 .50-EJO- w pervade 1 L157. 9 2 INVENTOR ATTORN EY Patented Nov. 11, 1930UNITED STATES PATENT OFFICE HARRY B. m, WESTIIELD, NEW JERSEY, ASSIGNOBTO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION 01' NEW YORKCONCENTBIC OONDUC'I'IN G SYSTEM Application fled 1:137 23, 1929. SerialIo. 865,514.

This invention relates to a conducting s stem for transmitting withsmall attenuation a band of frequencies whose upper limit extends wellabove the frequencies now em- 5 ployed in carrier transmission.

Modern developments in the art of communication render it highlydesirable to have available for transmission purposes a system whichwill transmit Without undue attenuation, frequencies extending from theaudio frequency range well up into the radio frequency range. Forexample, high grade circuits are now required for the transmission ofprograms over telephone lines to broadcasting stations. In order totransmit musical programs it is necessary to provide circuits that willtransmit a band of frequencies extendin well up toward 10,000 cycles, ascompared with the voice range ordinarily employed in telephon which didnot exceed about 2,500 cycles. or the best qual ty of transmission ofmusic it might be desirable to transmit over telephone lines frequenciesup to the audio limit, which would be in the neighborhood of 15,000 or20,000 cycles. Modern cable circuits are not adapted for transmission ofsuch high frequencies, and the only commercial circuit now availablewhich would be capable of transmitting frequencies of this order wouldbe the open wire circuits which have heretofore been employed for highfrequency multiplex. carrier transmission.

Even for carrier transmission, open wire lines have been founduncommercial for the transmission of frequencies much above 30,-

v 000 cycles. If, therefore, an open wire line is used for thetransmission of a high grade audio frequency program involvingfrequencies in the neighborhood of'15,000 or 20,000 cycles, the remanentfrequency range above such audio band would be so narrow as to be oflittle use for carrier transmission. From this standpoint, therefore, itwould be hi hly desirable to ha e available a clrcuit w ch comesdesirable to have available a conduct- "the same time available wouldtransmit without undue attenuation frequencies much higher than 30,000cycles.

The modern development of television also introduces a new factor.Existing television systems which have been experimented with have beenlimited to the transmission of a small image of a few square inches inarea, and in such image the elements of the picture making up the entireimage have been relatively large, so that the picture is not welldefined. The mechanical problems involved in designing a televisiontransmitting and receiving apparatus capable of picking up and receivingwith excellent definition a representation of some large scene, such asa ball game or a theatrical performance, are readily capable of solutionby known means, but the transmission of such a picture electricallyinvolves the transmission of frequencies from zero up to theneighborhood of 500,000 cycles, and there is no telephone circuit nowavailable which would commercially transmit any suchrange of frequenciesbecause of the enormous attenuation involved at frequencies above about30,000 cycles. It therefore being system which would transmit withoutundue attenuation a wide range of frequencies including the extremelyhigh frequencies necessary for television, the circuit being at ifnecessary for the transmission of a very large number of carrierchannels or for any desired number of bands of suflicient width for thetransmission, without undue distortion, of high grade programs.

In accordance with the present invention a circuit havin these desirablecharacteristics is obtained y employing concentric conductors ofrelativel large diameter, one acting as a return for tlie other. Theconductors may be insulated from each other and held in pro r concentricrelation by means of spaced was ers of some suitable dielectric of smallloss angle and low dlelactnc constant so as to introduce a minimumleakage loss between the conductors. By spacin the washers relativelyfar apart the principal part of the dielectric between the twoconductors will be air which, as is well known, involves sub-.

stantially no leakage loss. The conductors themselves may be made up ofstrips or ribbons of copper or other suitable conducting materialhelically wound to form a substantially continuous cylindrical shell.These strips or ribbons of conducting material are crimped at theiredges and otherwise pressed into the general form of the outer metallicprotective tape used in so-called B. X. cable. One ribbon is helicallywound about a suitable core with the edges of successive coils havingtheir crimped portions overlapping to form the inner conductor. Thedielectric washers are then mounted upon the inner conductor and anotherconducting tape or ribbon'similarly coiled upon the outer surfaces ofthe washers to form the outer concentric conductor. The wholeconstruction may then be covered externally by a suitable waterproofcovering such as is used in ordinary telephone cable or submarine cable.

A conducting system such as above outlined has a number of advantagessince it may be made waterproof, with the result thatthe leakage lossesbetween the conductors (which in the case of ordinary open wireconstruction .vary greatly with weather conditions and at highfrequencies contribute very substantially to the attenuation) may bemade small and constant. Furthermore, the increase in conductorresistance with frequency due to the skin eflect is relatively small, sothat the increase in the attenuation component due to resistance is muchless rapid than for ordinary open wire construction. Also, the form ofconstruction is such that interference from nearby circuits and noisecoming from external sources will be practically negligible. Moreover,the nature of the circuit is such that even though the outer conductorbe grounded it will not be subject to interference from ground currents.This enables the conductor system to be laid directly on the metallicsupports of an overhead cable or un-.

derground in,a conduit without any external insulation, even where thewaterproof covering is composed of a conductive lead or other metallicsheath. Also, the velocity of transmission will be uniform for allfrequencies.

Theinvention will now be more fully understood from the followingdescription when read in connection with the accompanying drawing, ofwhich Figures 1 to 7 are curves illustrating the principlesof theinvention; and Fig. 8 is a perspective view partially broken away of aportion of the conductor arrangement of the invention.

Referring to 1 Fig. 8 of the drawing, 10 designates an outer conductor,12 an inner conductor mounted concentrically therewith,

14 dielectric washers for separating the inner and outer conductors andmaintaining them in concentric relat1on,.15 a center coreupon which theinner conductor is mounted, and

In order to form the conductors, strips orv ribbons of conductingmaterial, such as copper, are pressed into the eneral shape of themetallic ta e employed or protecting socalled B. X. ca le. These stripsare crimped or formed at the edges as shown at 17, and one of the stripsis spirally wound upon the core 15 with its crimped edges overlapping,as

shown, to form the inner conductor 12. The dielectric washers 14 arethen mounted upon the inner conductor 12 in suitable spaced relation andanother tape or ribbon is similarly helically wound over the outersurfaces of the washers to form the outer conductor, the whole beingrendered waterproof by a suitable external covering.

In order that the attenuation may be made small at high frequencies theleakage loss between the conductors must be as small as possible. As theleakage loss is due to the nature of the dielectric interposed betweenthe conductors, the dielectric should be principally of air, since airintroduces no leakage loss. Accordingly, the spaced dielectric washers14 which hold the two conductors in proper concentric relation and outof electrical contact with each other, should be separated from eachother by suitable distances, and should be made as thin as possibleconsistent with the required mechanical strength. They should also becomposed of some dielectric of small loss angle and low dielectricconstant, since if these conditions are obtained the leakage loss (whichin the ordinary open wire system comprises a large part of theattenuation) may be made so small as to be practically negligible. Forexample, hard rubber, or preferably pyrex glass, may be used for theinsulating washers 14.-

In this connection it should be noted that as the outer shell may bemade watertight, the insulating washers will be maintained dry and freefrom dirt or contamination, so that the leakage loss will not increaseor change with time. In ordinary open wire construction where theinsulators are exposed to the air and to the action of the elements, theinsulators become coated with a film of relatively high resistanceconductive material which. introduces large leakage losses, and theseleakage losses are enormously increased be so small as to be negligible.It will be the outer'jconductor is grounded, it is posevident,therefore, that the use of spacing washers in a manner above describedintroduces substantially no leakage loss.

Since, as will be explainedlater, a conducting s stem of this type willbe practicall free rom external interference'even though sible to mountthe concentric conductor arrangement upon the metallic supports of anordinary overhead cable construction, or to permit the conductorarrangement to be uried directly in the ground or laid in a conduit suchas might be employed for under.- ground cable. The concentric conductorarrangement herein described is particularly adapted for these purposesas the helical arrangement of the conducting strips ermits.

of a flexible construction similar tot at obtaining in ordinary telehone cable so that it may readily. be pushe through the ducts of anunderground conduit.

As has been previously pointed out, no insulation between the outerconductor and any external conductor is necessary in order to preventinterference. The, insulation of the system, so far as it affectstransmission, is confined entirely to the space between the twoconcentric conductors. Consequently, b makin the external conductorwaterproo the lea age due to the dielectric of which the washer 14 iscomposed will not change with wet weather, and the surfaces of thedielectric washers will not deteriorate with time due to accumulationsof dirt or other foreign substances. The leakage loss of the system willtherefore be confined to that leakage loss which will be due to thedielectric material of which the washers are composed when the washersare new, clean and dry. If reasonably good dielectric material isemployed, the leakage loss due to the supporting washers will bepractically negligible, and if a material of very low loss angle anddielectric constant is 'used, such for example, as pyrex glass, thefactor of attenuation which is due to leakage will be so small as'to bepractically negligible. In ordinar open wire line construction (whichhas t e lowest attenuation at high frequencies of any type ofconstruction now employed in telephone practice) the attenuation due toleakage losses has been very large, and in wet weather becomes enormous.With the resent type of construction this factor of t e attenuationbecomes of little importance, and any attenuation due to this factor isfixed and unebangeable with variations in weather conditions.

In the ordinary type of conductor system,

either open wire or cable, where one solid wire acts as a return foranother solid wire,

the component of the attenuation which is a due to the skin effect is ofgreat importance at high frequencies. As is well known, where a'solidconductor is employed, as the frequency becomes hi her more and more ofthe current tends to ow at or near the surface of the conductor, so thatthe conductive ma- I terial near the center of the conductor takes butlittle part in the action at high frequencies. As a con uence, theconductor resistance increases wit frequency as a smaller and smallerpart of the cross-section of the conductor is usefully employed. If thesame amount of conductor material is arran d in the form of a relativelythin shell, t e resistance at any given high frequency is verfi muchreduced because now more nearly a of the material of the conductor isusefully employed in transmitting current. With a system of concentricconductors, such as described in connection with the present invention,bothconductors, being in the form of thin hollow shells, offer a muchless resistance at high frequencies due to-the skin efiect for the sameamount of conductlve material than in the case of an ordinarytransmission cir cuit consisting of two solid wires. In fact,

with a system of concentric conductors such as herein disclosed, thecurrent at higher frequencies tends to flow more and more at the innersurface of the outer conductor and the outer surface of the innerconductor, due to the well known proximity efiect. The result is thatwhile that component of the attenuation which is due to the conductorresistance increases with freqllliency, the rate of increase is verymuch less t an in the case of an open wire line. By means of theconstruction above described, therefore, we have the one component ofthe attenuation which is due to leakage losses or the so-called shunteffect reduced to practically negligible proportions by reason of thefact that thedielectric between the conductors is very largely of airand such other dielectric as is employed introduces but little leakage,while the other component of attenuation, namely,

that due to the conductor resistance or socalled series efiect is verymuch reduced as compared with the ordinary type of conducting system forany given frequency.

The form of construction herein disclosed also has the advantage that itdoes not'produce material interference in the neighboring circuit and,conversely, is substantially free from interference from nearby circuitsand noise coming from external sources.

In order to understand this more clearly it should be remembered thatthe interference between any two circuits is due to the fact that theone circuit lies within either the electric field or the magnetic field,or both, of the other circuit. Considering first the magnetic field, letus consider two conductors a and .b circular in cross-section andarranged side by side, one acting as a'return for the other. Theseconductors are shown in section in Fig. 2. The lines of force due to themagnetic field surround each conductor and are crowded together in thespace between the two conductors. Any other conducting system introducedat a point where the conductors of such other system will be cut bythese lines of force will have induced therein cross-talk from theconductor system 0-6. If, now, we have two conductors 10 and 12, asshown in Fig. 1, in the form of hollow shells concentrically arrangedand the one acting as a return for the other, each conductor has linesof magnetic force surrounding it, each successive line of force being oflarger radius and all of the lines, due to the current flo ing in theparticular conductor, such as 12, ing external thereto. As the currentflows in one direction through the conductor 12 and in the oppositedirection through the conductor 10, the lines of magnetic force due tothe current through the conductor 12' are in one direction, as indicatedby the arrows. while those due to the current flowing in the conductor12 are in' the opposite direction. Now, an ins ection of Fig. 1' showsthat some of the lmes of force due to the current in the conductor 12are within the conductor 10, but none are within the conductor 12. Onthe other hand, all of the lines of force due to the current flowing inthe conductor 10 are external to said conductor, and the two magneticfields produced by the currents flowing in the two conductors tend tooppose each other outside of the conductor 10. The resultant field ofmagnetic force external to the conductor 10 is, therefore. very small,and the only effective magnetic field lies within the space between thetwo conductors. Since the external magnetic field is very small it isobvious that another conductive system external to the conductor 10 willnot receive any appreciable amount of cross-talk interference from theconducting system 1012.

In so far as the electric field is concerned, the distribution of thefield in the case of two parallel conductors a and b is as indicated inFig. 4, so that any external conductor which is cut by the lines ofelectric force between a and b wil have cross-talk induced therein. Inthe case of the two concentric conductors 1012, however, the electricfield set up due to currents flowing in the two conductors is entirelybetween the adjacent surfaces of the two conductors, as indicated inFig. 3. No external conductor can possibly be cut by any of the lines ofthe electric field due to current flowing in the conductor 12 andreturning in the conductor 10, or vice versa, and hence .so far as theelectric field is concerned, no possible external interference can takeplace.

The concentric arrangement not.only has the advanta e that it producessubstantially no external field to interfere in other circuits, but itis practically free from interference due to any external source. Forexample, referring to Fig. 5, let us assume that some external forceproduces a field as represented by the arrows. The lines of forcecutting the two concentric conductors produce differences in potentialbetweenv points of the two conductors. For example, consider the points0 and d, the one on the outer surface of the conductor 12 and the otheron the inner surface of the conductor 10. The lines of force cutting thetwo conductors produce an induced E. M. F. between these points in thedirection and having the value indicated by the arrow c-ri. Since thesame number of lines of force out the two conductors on the oppositeside of the diagram, a difference in potential indicated by the arrowc'0l' will be produced between the two points 0 and d. The inducedpotential cd, however, tends to produce a current fiow equal to andopposite that induced by the difference of potential at c'd, so that abalance is obtained. Due to the symmetry of the conducting system withrespect to the cutting lines of force, all differences in potentialinduced between any other two points of the two conductors will bebalanced by similar difi'erences of potential induced at correspondingpoints on the opposite side, so that if the interfering field is evenlydistributed through the cross-scectional area of the conducting system(as would be the case where the interfering source is not too near thesystem sub stantially no interfering efiect would result in theconducting system 10-12.

While the foregoing explanation only applies to fields perpendicular tothe axis of the conducting system, field components parallel to the axisare also prevented from causing interference. This is because the skineffect in the outer conductor furnishes pro- -tection against suchfield.

As has been previously stated, the concentric conducting system is freefrom external interference even though the outer conductor be grounded,and hence there is no necessity for insulating the outer conductor frommetallic supports in case it is mounted like an overhead cable, or fromground in case it is placed in a conduit. The reason for this is that aground return circuit is noisy, due to the fact that a wire supportedabove ground forms with the ground a loop to pick up stray fields. Butfrom the diagram of Fig. 5 it is evident that if the outer conductorsuch as 10 is grounded so that it in effect becomes a ground return forthe conductor 12, it is only the space between the two -concentricconductors that acts as the loop to pick up stray fields. Hence, as hasbeen just exp ained in connection with Fig. 5, substantially nointerfering currents are induced in the conductors 10-12.

In order that a conducting system such as herein disclosed ma have assmall attenuation as possible at igh frequencies, the diameters of thetwo concentric conductors should be made as large as ossible. However,due to practical consi erations it may be desirable that the systemshould be of such character that it might be used in existing cableducts or in connection with present aerial cable construction. For thesereasons, in practice it is convenient to make the dimeter of theexternal conductor not much greater 'than about two and five-eighthsinches, if the conductor is to be used in the existing telephone plant.For mechanical reasons the thickness of the conductors should be made assmall as is consistent with securing ro r values of electricalresistance and mac anical stren h. In general, it has been found that ife concentric conductor is made thick enough to satisfy the mechanicalrequirements, the electrical resistance is not a limiting factor in theattenuation at high frequencies. This is due to the skin effect orproximity effect which, as previously described, causes the current tocrowd to the outer surface of the inner conductor and the inner surfaceof the outer conductor as the frequency increases, thereby rendering theremaining cross-sectional area of little utility for carrying current.

As the outer conductor is or at least can be made watertight, thelealrage losses can be reduced to very low values by the use of pyrexinsulation where mechanical support is necessary, with the largestpossible air space between the two conductors. Under these circumstancesthe leakage loss will not change with weather conditions. For zeroleakage (a condition which would be approximately obtained) theattenuation equals R/2 /zl/L at high frequencies, where represents theresistance, 0 the capacity and L the inductance. From this expression itis evident that the values of R and C should be as small as possible. Athigh frequencies R is inversely proportional to the diameter of theconductor, and hence the attenuation will be smaller at any givenfrequency the larger the diameter of the conductor. The capacity C alsois an inverse function of the diameter and decreases as the differencebe tween the diameters of the inner and outer conductors increases.Consequently, if the diameter of the outer conductor is fixed, as thediameter of the inner conductor increases om some small value, theresistance of the conducting system decreases, while at the same timethe capacity increases. The decrease in resistance tends to reduce theattennation while the increase in capacity tends to increase theattenuation. For a given diameter of the inner conductor these twoeffects balance and the attenuation becomes a minimum.

Fig. 6 is a curve showing how theattenuation varies with diameter of theinner conductor at 500,000 cycles, with the diameterof the outerconductor fixed at two and onehalf inches. This curve shows a minimumattenuation of .43 transmission units per mile for an inner conductordiameter of about .7 inch. As will be clear from the curve, either anincrease or decrease of the diameter of the inner conductor from theforegoing value results in an increase in the attenuation. In Fig. 7 isshown a curve of the attenuation at various frequencies of a concentricconductor system whose outer conductor has a diameter of two andone-half inches'and the inner conductor has the optimum diameter ofabout .7 inch. It will be observed from this curve that while theattenuation increases with fre uency, the slope of the curve is notsteep1 an the increase in attenuation is very muc less than would be thecase for an open wire line.

At 500,000 cycles the attenuation r mile of-a 165-gau open wire circuitwit a spacing of 12 inc es between wires is 1.67 transmission units,which compares with .43 transmission units per mile for the concentricconductor system. The advantages of usingthe latter are even greaterthan would a pear from these fi res on account of the lbwer levels towhic the current may be attenuated before a repeater is necessary. Thisis due to the absence of coupling to external fields and results in avery low noise level. On an open wire circuit the level could not beallowed to go below transmission units, while with the concentricconductor system it might be permitted to fall as low as transmissionunits. If the repeaters are adjusted to give an output of +10transmission units, this would result in a repeater spacing of 36 milesfor the open wire circuit and 210 miles for the concentric returncircuit. It appears to be impractical to devise a transmisslon systemfor an open wire circuit at such high frequencies, and the highfrequency cross-talk would limit its use to one circuit on a given lead.Due to the absence of couplings to other circuits this limitation wouldnot apply to the concentric conductor system, and any desired number ofsuch conductor systems might be mounted upon the same pole line orcarried in adjacent conduits without undue interference.

. It follows, therefore, for the transmission of frequencies up to500,000 cycles an open wire circuit would be quite unsuitable, whereasthe concentric conductor system would carry frequencies as high as1,000,000 to 2,000,000 cycles or even higher, without undue attenuation.A carrier telephone system I could be operated over such a conductorwith as many as one to two hundred two-way channels, allowin 5,000cycles for each channel in each directlon. This is comparable to thenumber of circuits which might be obtained from the pairs of wires in acable ofequivalent size. Any particular circuit in the cable could notbe used for the transmission of frequencies much above the ordinarytelephone'range, and hence could not be employed for the transmission ofmusicalprograms involving frequencies up to the audio limit withoutusing a very expensive loading system. A cable circuit could notconceivably be loaded to transmit frequencies high enough for 00dtelevision transmission. The concentrlc conductor system, on the otherhand, may be employed for either program transmission or television.

It will be obvious that the general rinciples herein disclosed may beembo ied in many other organizations widely different from thoseillustrated without departin from the spirit of the invention as definein the following claims.

What is claimed is:

1. In a conducting system for the communicationof intelligence, twoconductors connected one as a return for the other, each conductor beingin the form of a shell of conductive material and the two conductorsbeing arranged concentrically one inside the other, means to preventmoisture from entering the interior of the outer conductor, the innerconductor being formed by spirally winding a thin strip of conductingmaterial so that the edges of successive spirals overlap, and insulatingmeans for separating the conductors electrically and for maintainingthem in concentric relation,-

said insulating means being so formed that the dielectric betweenadjacent surfaces of the conductors will be principally gaseous, andsaid outer conductor bein formed by spirally winding a thin strip 0conducting material over said insulating means so that the edges ofsuccessive spirals overlap.

2. In a conducting system for the communication of intelligence, twoconductors connected one as a return for the other, each conductor beingin the form of a shell of conducting material and having a diameterlarge as compared with its wall thickness so that its attenuation willbe relatively small at high frequencies, said conductors being arrangedconcentrically one inside the other, means to prevent moisture fromentering the interior of the outer conductor, the inner conductor beinformed by spirally winding a thin strip 0 conducting material so thatthe edges of successive spirals overlap, and insulating means forseparating the conductors electrically and for maintaining them inconcentric relation, said insulating means being so formed that thedielectric will be principally gaseous, and said outer conductor beingformed by spirally windin a thin strip of conducting material over saidinsulating means so that the edges of successive spirals overlap.

3. In a conducting system for the communication of intelligence, twoconductors connected one as a return for the other, each conductor beingin the form of a shell of conductive material and the'two conductorsbeing arranged concentrically one inside the other, means to preventmoisture from en-,

adjacent surfaces of the conductors will be principally gaseous, andsaid outer conductor being formed by spirally winding a thin betweenadjacent surfaces of the conductors strip of conducting material withcrimped edges over said insulating means so that the crimped edgesoverlap.

4. In a conducting system for the communication of intelligence, twoconductors connected one as a return for the other, each conductor beingin the form-of a shell of conducting material and having a diameterlarge as compared with its wall thickness so that its attenuation willbe relatively small at'high frequencies, said conductors being arrangedconcentrically one inside the other, means to prevent moisture fromentering the interior of the outer conductor, the inner conductor beingformed b spirally windin a thin strip of conductmg material wit crimpededges so that the crimped edges of successive spirals overlap, andinsulating means for separating the conductors 'electrically and formaintaining them in concentric relation, said insulating means being soformed that the dielectric between adjacent surfaces of the conductorswill be principally gaseous, and said outer conductor being formed byspirally winding a thin strip of conducting material with crimped edgesover said insulating means so that the crimped edges overlap.

5. In a conducting system for the communication of intelligence, twoconductors connected one as a return for the other, the outer conductorbeing in the form of a shell of conductive material and the twoconductors being arranged concentrically one inside the other, means toprevent moisture from entering the interior of the outer conductor, andinsulating means for separating the conductors electrically and formain-.

electric between adjacent surfaces of the conductors will be principallygaseous, and said outer conductor being formed by spirally winding athin strip of conducting material over said insulatlnf means so that theed es of successive spira s overlap.

n testimony whereof, I have signed my name to this specification this20th day of May, 1929.

-HARRY R. NEIN.

